The war of the sexes is particularly hellish for many promiscuous insects. Male fruit flies, for example, not only have to battle other suitors, but their reproductive systems must continuously evolve to outwit that of the female. Now, scientists think this randy competition may lead to the birth of new species.
The conflict arises when females mate with more than one male at about the same time. In such a ménage à beaucoup, sperm of several suitors are competing to fertilize. This rivalry has led some male flies to evolve some nasty tactics. They try to outdo other suitors by spiking seminal fluid with proteins that kill other flies' sperm--with collateral damage to the female. Other males target the female directly with proteins that limit her sexual desire, reducing the chance that a future rival will supplant the male's sperm. For protection, female flies have evolved tricks of their own (ScienceNOW, 7 January 1999). Göran Arnqvist of the University of Umeå, Sweden, reasoned that these kinds of tactics might eventually make one population of flies unable to mate with another--creating a new species.
Arnqvist and his colleagues tested their idea by compiling behavioral and genetic information on thousands of insects, including species of flies, mosquitoes, beetles, and butterflies. The researchers were trying to find pairs of groups with a common ancestor, in which one group engaged in conflict between the sexes, while the other didn't. To gauge how sexual conflict influenced the rate of speciation, the researchers simply counted the number of species within each group.
The results were dramatic, Arnqvist says. Speciation had taken place four times as fast in clades where females mated with many males, his team reports in the 12 September Proceedings of the National Academy of Sciences. The difference remained even when the researchers accounted for factors like the size of an insect's geographic range, which might partly account for an elevated speciation rate.
The research identifies a potent mechanism--rapid evolution of the reproductive system--involved in speciation, says William Rice, an evolutionary biologist at the University of California, Santa Barbara. "There's been a lot of work, but not a lot of progress until quite recently," he says. Rice thinks sexual conflict may also occur in vertebrates, but few people have checked.